This invention relates to electrical control circuitry and more particularly to electrical control circuitry particularly adapted for use in burner control systems.
Burner control systems are designed both to monitor the existence of flame in the supervised combustion chamber and to time sequences of operation of burner controls. Safety of burner operation is a prime consideration in the design of burner control systems. For example, if fuel is introduced into the combustion chamber and ignition does not take place within a reasonable time, an explosive concentration of fuel may accumulate in the combustion chamber. The burner control system should reliably monitor the existence of flame in the combustion chamber, accurately time a trial-for-ignition interval, inhibit ignition if a false flame signal is present, and shut down the burner in safe condition whenever a potentially dangerous condition exists. Examples of such burner control systems are disclosed in my U.S. Pat. No. 3,840,322.
Among the considerations in burner control system design are reliability of operation, manufacturing cost, the provision of precise timing cycles (particularly those of short duration), and the nature of the response of the burner control to a flame failure condition after flame has been established, for example, an immediate shut down of the burner system, an immediate attempt to re-establish flame, or an attempt to re-establish flame only after a pre-ignition (purge) interval.
In burner control systems as above-identified and particularly those disclosed and claimed in applicant's referenced copending application, one problem that has been encountered involves the problem of extinguishing the pilot flame upon application of main fuel supply to the combustion chamber. This problem is particularly aggravated if main fuel supply is initiated during the time when the pilot flame is not fully established in the stabilized condition and the electric ignition spark is removed at the time of application of main fuel. The prior art attempts to solve this problem have been relatively complex and expensive and do not provide the desired reliability at a cost and complexity level which is consistent with providing this feature to the full range of size for burner installations.